Medical Genetics 2. Trisomy 18 Trisomy 18 was independently described by edwards et al and smith et al in 1960. Among liveborn children trisomy 18 is the second most common autosomal trisomy after trisomy 21. The disorder is characterized by severe psychomotor and growth retardation microcephaly microphthalmia, malformed ears, micrognathia or retrognathia, microstomia, distinctively clenched fingers and other congenital malformationsMedical Genetics 2. Trisomy 18 Trisomy 18 was independently described by Edwards et al and Smith et al in 1960. Among liveborn children, trisomy 18 is the second most common autosomal trisomy after trisomy 21. The disorder is characterized by severe psychomotor and growth retardation, microcephaly, microphthalmia, malformed ears, micrognathia or retrognathia, microstomia, distinctively clenched fingers, and other congenital malformations